The present invention relates to a mixing device for extruders comprising a substantially cylindrical body to be terminally engaged with an extrusion screw having a shank and a transportation thread helically wound on the shank; and at least one primary mixing thread helically wound at least partly around the substantially cylindrical body in a predetermined winding direction and having a predetermined crest diameter.
1. Field of the Invention
It is known that in the plastic-material field as regards working and production of articles of manufacture, rotating-screw extruders are used into which the plastic material in granular form is introduced to obtain a compound at the exit which is adapted for subsequent drawing or injection operations in appropriate die-casting molds or others. In many cases the extruder is part of a so-called “plasticizing unit” directly connected to a mold, a rolling set or other type of device for shaping or forming the plastic material delivered from the extruder.
2. Description of the Prior Art
Extruders consist of a cylindrical body within which a screw is axially mounted which is set in rotation by appropriate motors. The bottom diameter of the screw thread grows along the axial extension of the screw starting from an inlet opening towards an outlet opening; in addition the cylinder walls are provided with appropriate heating resistances. The material moving forward in the race of the screw thread is submitted to temperature and pressure rise as needed for obtaining the required physical properties.
One of the main problems typical of the extrusion screws of the traditional type is an insufficient homogeneity of the material that adversely affects the final product obtained by drawing or molding.
For example, since heat is supplied through the cylinder walls, the presence of a temperature variation in the thickness of the material included between the screw web and the inner wall of the cylinder has been noticed. This temperature variation corresponds to a variation in the density of the material itself. Density variation in the material may give rise to an inhomogeneous distribution of the material in the mold, for example.
In addition, the polymer molecules composing each resin tend to remain mutually connected to form the so-called “memory lattices” in which the polymer molecules keep mutually bonded giving the compound a clotty appearance, in which small compact material clusters immersed in a more fluid current are identifiable.
Finally, when resins of different color and nature are processed, an incomplete mixing of same leads to production of articles of manufacture in which regions of different color are recognizable due to the prevalence of a resin with respect to the other.
To partially solve the above problems, extrusion screws are known that are provided, at the screw end close to the outlet, with a mixing device the function of which is to improve homogeneity of the material and make temperature distribution more uniform.
For example, document U.S. Pat. No. 5,318,357 shows an extrusion screw having a mixing portion provided with a plurality of helical primary threads interrupted at intervals along the extension of same. The distinct threads are mutually connected by plates circumferentially extending around the screw web and defining regions of reduced depth in the races confined by the primary threads.
The Applicant has found that the mixing level reached by the devices of the known art can be further improved.